a49559befb
Current SCEV expansion will expand SCEV as a sequence of operations and doesn't utilize the value already existed. This will introduce redundent computation which may not be cleaned up throughly by following optimizations. This patch introduces an ExprValueMap which is a map from SCEV to the set of equal values with the same SCEV. When a SCEV is expanded, the set of values is checked and reused whenever possible before generating a sequence of operations. The original commit triggered regressions in Polly tests. The regressions exposed two problems which have been fixed in current version. 1. Polly will generate a new function based on the old one. To generate an instruction for the new function, it builds SCEV for the old instruction, applies some tranformation on the SCEV generated, then expands the transformed SCEV and insert the expanded value into new function. Because SCEV expansion may reuse value cached in ExprValueMap, the value in old function may be inserted into new function, which is wrong. In SCEVExpander::expand, there is a logic to check the cached value to be used should dominate the insertion point. However, for the above case, the check always passes. That is because the insertion point is in a new function, which is unreachable from the old function. However for unreachable node, DominatorTreeBase::dominates thinks it will be dominated by any other node. The fix is to simply add a check that the cached value to be used in expansion should be in the same function as the insertion point instruction. 2. When the SCEV is of scConstant type, expanding it directly is cheaper than reusing a normal value cached. Although in the cached value set in ExprValueMap, there is a Constant type value, but it is not easy to find it out -- the cached Value set is not sorted according to the potential cost. Existing reuse logic in SCEVExpander::expand simply chooses the first legal element from the cached value set. The fix is that when the SCEV is of scConstant type, don't try the reuse logic. simply expand it. Differential Revision: http://reviews.llvm.org/D12090 llvm-svn: 259736 |
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README.txt
Low Level Virtual Machine (LLVM) ================================ This directory and its subdirectories contain source code for LLVM, a toolkit for the construction of highly optimized compilers, optimizers, and runtime environments. LLVM is open source software. You may freely distribute it under the terms of the license agreement found in LICENSE.txt. Please see the documentation provided in docs/ for further assistance with LLVM, and in particular docs/GettingStarted.rst for getting started with LLVM and docs/README.txt for an overview of LLVM's documentation setup. If you are writing a package for LLVM, see docs/Packaging.rst for our suggestions.